Malic enzymes catalyze the oxidative decarboxylation or malate to form pyruvate and CO2. Two types of these enzymes are known to occur in animal cells. Both cytosol and mitochondria may contain a strictly NADP-dependent form. Mitochondria only may also contain a NAD(P)-linked enzzme. The latter enzyme, which we have purified about 100-fold from adrenal cortex mitochondria, is an allosteric enzyme both electrophoretically and kinetically different from the NADP-linked forms. Mitochondria from most normal tissues contain either both NAD(P)-and NADP-dependent forms or only the NADP-dependent malic enzyme. Mitochondria from ascites tumors, however, contain only the allosteric NAD(P)-dependent enzyme. Moreover, this enzyme has a higher specific activity in tumor mitochondria compared to normal mitochondria. All ascites tumors that we have examined are deficient in the mitochondrial NADP-linked malic enzyme. They differ, therefore, from normal cells in this important respect, as well. A regulatory NAD(P)-dependent malic enzyme in tumor mitochondria would augment the oxidative pathway of malignant cells. For example, this enzyme would serve as a pyruvate source for continued citric acid cycle activity during glutamine and glutamate oxidation.